Experimental Investigation of Local Displacement Speeds of Wrinkled Unsteady Flames

Local flame displacement speeds were measured along the flame front of laminar unsteady premixed flames wrinkled by laminar toroidal vortices. The displacement speed is argued to be the most important and sensitive parameter that must be simulated correctly in numerical simulation of turbulent flames. An axisymetric flame wrinkle is created in order to measure all components of the normal velocity vector and stretch; particle image velocimetry (PIV) and high-speed shadowgraph cinematography yield the difference between the interface velocity and the reactant gas velocity - the displacement speed. This repeatable flame-vortex interaction problem provides a useful test data to assess direct numerical simulation models and flame stretch theory. Lean and rich methane-air, and lean propane-air flames were investigated to assess stable and unstable preferential diffusion effects on local displacement speeds. The strength of the laminar toroidal vortices was varied from 1.4 to 10 times the unstretched laminar flame burning velocities, SL. Results showed that the local displacement speeds follow the trends predicted by the laminar steady-state theory, but not the magnitudes. Large variations in local displacement speed measurements were found, ranging from -6 to 10 times SL. The negative values occur at the two locations that the steady-state theory predicts low values. Planar laser induced fluorescence of the OH radical was used successfully to locate the flame front boundary in the velocity fields obtained from the PIV images.

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